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| United States Patent |
6,131,797
|
|
Gasdaska
, et al.
|
October 17, 2000
|
Method for joining ceramic to metal
Abstract
Metal and ceramic members are joined together by a braze joint including a
member made of bar stock molybdenum, a first ductile member brazed between
the molybdenum member and the metal member, and a second ductile member
brazed between the molybdenum member and the ceramic member. The braze
joint may be used to join a ceramic wheel shaft to a metal shaft in a
turbomachine. The braze joint allows torsion to be transmitted between the
ceramic wheel shaft and the metal shaft without the use of a metal sleeve.
| Inventors:
|
Gasdaska; Charles (Ogdensburg, NJ);
Limoncelli; Edward V. (Clinton, CT)
|
| Assignee:
|
AlliedSignal Inc. (Morristown, NJ)
|
| Appl. No.:
|
193031 |
| Filed:
|
November 16, 1998 |
| Current U.S. Class: |
228/122.1; 228/124.7; 228/189; 228/262.7 |
| Intern'l Class: |
B23K 035/28; B23K 031/12 |
| Field of Search: |
228/122.1,124.7,141.1,173.5,164,188,189
|
References Cited
U.S. Patent Documents
| 3921616 | Nov., 1975 | Kish | 125/11.
|
| 4606978 | Aug., 1986 | Mizuhara | 428/606.
|
| 4723862 | Feb., 1988 | Ito et al. | 403/272.
|
| 4798320 | Jan., 1989 | Fang.
| |
| 4989773 | Feb., 1991 | Ishiyama | 228/122.
|
| 4991991 | Feb., 1991 | Ito et al.
| |
| 5001019 | Mar., 1991 | Ito et al.
| |
| 5073085 | Dec., 1991 | Ito et al.
| |
| 5076484 | Dec., 1991 | Ito et al.
| |
| 5186380 | Feb., 1993 | Beeferman et al. | 228/121.
|
| 5234152 | Aug., 1993 | Glaeser | 228/121.
|
| 5340012 | Aug., 1994 | Beeferman et al. | 228/56.
|
| 5372298 | Dec., 1994 | Glaeser | 228/195.
|
| 5461898 | Oct., 1995 | Lessen | 72/256.
|
Other References
S. L. Semiatin. Metals Handbook: Ninth Edition vol. 13; Forming and Forging
237-238.
|
Primary Examiner: Ryan; Patrick
Assistant Examiner: Johnson; Jonathan
Attorney, Agent or Firm: Zak, Jr., Esq.; William J.
Claims
What is claimed is:
1. A method of joining a metal member to a ceramic member, the method
comprising the steps of:
interposing an interlayer between joinable surfaces of the metal and
ceramic members, the interlayer including a molybdenum member and first
and second ductile members, the molybdenum member being located between
the metal and ceramic members, the first ductile member being located
between the molybdenum member and the metal member, the second ductile
member being located between the molybdenum member and the ceramic member,
the molybdenum member being of a bar stock type;
joining the first and second ductile members to the molybdenum member; and
brazing the first and second ductile members to the metal and ceramic
members, respectively.
2. The method of claim 1, wherein the first and second ductile members are
joined to the molybdenum member by brazing, and wherein the joining and
the brazing both include heating steps that are performed at the same
time.
3. The method of claim 2, wherein the joining and the brazing are performed
by the steps of:
placing a first braze between the first ductile member and the joinable
surface of the metal member;
placing a second braze between the first ductile member and the molybdenum
member;
placing a third braze between the molybdenum member and the second ductile
member;
placing a fourth braze between the second ductile member and the joinable
surface of the ceramic member; and
heating the interlayer and the first, second, third and fourth brazes
together at a brazing temperature.
4. The method of claim 3, wherein the ductile members are made of nickel,
and wherein each braze includes silver, copper and titanium.
5. The method of claim 4, wherein the first and fourth brazes each include
about 69% silver, about 27% copper and about 4% titanium by weight, and
wherein the second and third brazes each include about 71% silver, about
27.5% copper and about 1.5% titanium by weight.
6. The method of claim 4, wherein the first braze has a pre-brazing
thickness of about 0.003 inches, the second and third brazes each have a
pre-brazing thickness of about 0.008 inches, and the fourth braze has a
pre-brazing thickness of about 0.006 inches.
7. The method of claim 1, wherein a crystal axis of the molybdenum member
is perpendicular to the joinable surfaces of the metal and ceramic
members.
8. The method of claim 1, wherein the molybdenum member has a thickness of
at least 0.005 inches.
9. The method of claim 1, wherein the molybdenum member is nickel-plated.
10. The method of claim 9, wherein nickel plating on the molybdenum member
has a thickness between 0.000050 and 0.000100 inches.
11. A combination comprising:
a metal member having a joinable surface;
a ceramic member having a joinable surface; and
a braze joint including a layer of bar stock type molybdenum between first
and second layers of ductile material, the first ductile layer being
brazed to the metal, the second ductile layer being brazed to the ceramic,
a crystal axis of the molybdenum member being perpendicular to the
joinable surfaces of the metal and ceramic members.
12. The combination of claim 11, wherein the first ductile layer is joined
to the metal by a first braze having an after-brazing thickness of between
about 0.003 and 0.006 inches; wherein the first and second ductile layers
are joined to the molybdenum layer by second and third brazes having an
after-brazing thickness of between about 0.005 and 0.008 inches; and
wherein the second ductile layer is joined to the ceramic by a fourth
braze having an after-brazing thickness of about 0.006 to 0.010.
13. The combination of claim 11, wherein the molybdenum member has a
thickness of at least 0.005 inches.
14. The combination of claim 11, wherein the ductile members are made of
nickel.
15. A turbomachine comprising:
a ceramic rotating component;
a metal rotating component; and
a braze joint joining the ceramic component to the metal component, the
braze joint including a member made of bar stock type molybdenum, a first
ductile member brazed between the molybdenum member and the ceramic
component, and a second ductile member brazed between the molybdenum
member and the metal component;
whereby the braze joint allows torque loads to be transmitted between the
ceramic and metal rotating components.
16. The turbomachine of claim 15, wherein the first ductile member is
joined to the metal component by a first braze having a thickness of
between about 0.003 and 0.006 inches; wherein the first and second ductile
members are joined to the molybdenum member by second and third brazes
having a thickness of between about 0.005 and 0.008 inches; and wherein
the second ductile member is joined to the ceramic component by a fourth
braze having a thickness of about 0.006 to 0.010.
17. The turbomachine of claim 15, wherein a crystal axis of the molybdenum
member is perpendicular to the joinable surfaces of the metal and ceramic
members.
18. The turbomachine of claim 15, wherein the molybdenum member has a
thickness of at least 0.005 inches.
19. The turbomachine of claim 15, wherein the ductile members are made of
nickel.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to a method of joining ceramic to
metal and specifically to a method of creating a ceramic-metal interface
that allows torsion loads to be transmitted between a ceramic member and a
metal member. The invention also relates to turbomachinery including metal
and ceramic rotating components that are joined together.
One of the more difficult problems of joining ceramic to metal is
overcoming the large difference in thermal expansion between the metal and
the ceramic. In particular, a ceramic such as silicon nitride has a low
thermal expansion as compared to a steel or nickel alloy.
Interlayer materials including tungsten alloys have been used to overcome
the difference in thermal expansion. The interlayer materials reduce
residual stresses produced by the large, differences in thermal shrinkage
upon cooling after brazing.
In an automotive turbocharger including a ceramic turbine wheel that is
brazed to a metal shaft, a metal sleeve is used to cover the braze joint.
The metal sleeve is typically made of a special low thermal expansion
metal alloy that avoids the introduction of unwanted residual stresses in
the ceramic. The metal sleeve protects the braze joint from high
temperatures, and it protects the braze joint from cracking under high
bending and twisting loads. The metal sleeve also provides a sealing
surface. (A ceramic shaft of the turbine wheel extends from a hot side of
the turbocharger, through a seal, to a cooled side. However, it is
undesirable to form a seal on a brittle ceramic shaft.) Additionally, the
metal sleeve itself can provide additional bonding, as is the case where
the ceramic shaft is press-fitted into the metal sleeve.
However, special metal alloys used for the metal sleeve are expensive and
they are not readily available. Furthermore, precision machining of the
ceramic and metal mating surfaces is performed to avoid the introduction
of unwanted stresses and, in the case of press-fitting, to ensure that the
mating surfaces remain in contact throughout the operating temperature
range of the machine. The sleeve also makes a post-brazing inspection of
the braze joint difficult to perform. These problems are especially
important to mass production items such as automotive turbochargers.
Furthermore, brazing is often performed in two steps when a metal sleeve is
involved. The addition of a second step adds to the cost and complexity of
joining the metal to the ceramic.
Even when a metal sleeve is not involved, some brazing operations are
performed in two steps. The metal is brazed separately from the ceramic to
prevent a migration of braze material between layers that might otherwise
occur during brazing of multiple materials. The migration could cause a
change in composition and properties of the braze and thereby have a
deleterious affect on strength of the joint.
There is a need to join metal to ceramic in a single brazing step. In
applications involving the transmission of large torsion loads, there is
also a needed to create a high strength, sleeveless braze joint.
SUMMARY OF THE INVENTION
The present invention may be regarded as a braze joint that allows torsion
loads to be transmitted between metal and ceramic turbomachine components.
An interlayer is placed between a metal member and a ceramic member. The
interlayer includes ductile members and a molybdenum member of the bar
stock type. The ductile members are joined (e.g., brazed) to the
molybdenum member, and the ductile members are brazed to the metal and
ceramic members. If the ductile members are also brazed to the molybdenum
member, the braze joint can be formed in a single brazing step. The braze
joint is sleeveless, which eliminates the need for low-expansion sleeve
metals that are expensive and hard to procure.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of a metal member, a ceramic member and an
interlayer prior to brazing;
FIG. 2 is a flowchart of a method of joining the metal member, the ceramic
member and the interlayer;
FIG. 3 is an illustration of a braze joint after brazing, the braze joint
having been created in accordance with the method shown in FIG. 2; and
FIG. 4 is an illustration of a turbomachine including a ceramic wheel that
is joined to a metal shaft in accordance with the method shown in FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Reference is made to FIGS. 1 and 2, which illustrate the joining of a metal
member 10 to a ceramic member 12. The surfaces of the members 10 and 12 to
be joined will be referred to as "joinable" surfaces.
An interlayer is placed between the joinable surfaces of the metal and
ceramic members 10 and 12 (block 102). The interlayer includes a
molybdenum member 14 and first and second ductile members 16 and 18 made
of nickel. The first nickel member 16 is located between the molybdenum
member 14 and the joinable surface of the metal member 10. The second
nickel member 18 is located between the molybdenum member 14 and the
joinable surface of ceramic member 12.
The nickel members 16 and 18 each have thickness between 0.005 inches and
0.100 inches (e.g., 0.030 inches). Each nickel member 16 and 18 may have
the same diameter as the metal and ceramic members 10 and 12.
The molybdenum member 14 may be machined from molybdenum bar stock, such as
that corresponding to ASTM B387-90. Bar stock molybdenum, which is
extruded, has crystals that are elongated in a preferred orientation.
Thus, the molybdenum member 14 has crystals 15 that extend along a
preferred orientation or crystal axis A (the size of the crystals 15 in
FIG. 1 is exaggerated merely to illustrate the preferred crystal
orientation). The crystal axis A of the molybdenum member 14 is
perpendicular to the joinable surfaces of the metal and ceramic members 10
and 12, as shown in FIG. 1. Such an orientation of the crystal axis A
minimizes the stress (caused by bending and twisting) at the crystal
boundaries of the molybdenum member 14. Thus, the crystals 15 of the
molybdenum member 14 are aligned in an orientation A that reduces the
chances of separation (cracking) due to twisting and bending.
The molybdenum member 14 may have a thickness of at least 0.005 inches.
Maximum thickness of the molybdenum member 14 is limited by practical
considerations (e.g., size constraints imposed by a turbomachine). The
molybdenum member 14 may have the same diameter as the metal and ceramic
members 10 and 12. The molybdenum member 14 may also be cleaned and plated
with nickel metal having a thickness between 0.000050 and 0.000100 inches.
Brazes 20 to 26 are then placed between the metal and ceramic members 10
and 12 (block 104). A first braze 20 is placed between the metal member 10
and the first nickel member 16; a second braze 22 is placed between the
first nickel member 16 and the molybdenum member 14; a third braze 24 is
placed between the molybdenum member 14 and the second nickel member 18;
and a fourth braze 26 is placed between the second nickel member 18 and
the ceramic member 12. Each braze 20 to 26 includes copper and silver.
An active metal such as titanium may be included in each braze 20 to 26.
For example, the titanium may be coated onto a silver-copper braze foil.
Although the titanium is not required for metal-to-metal brazing, there
are advantages to using the titanium in metal-to-metal interfaces and
ceramic-to-metal interfaces. The titanium provides good wetting and
bonding. In contrast, titanium-free brazes that are brazed in a single
assembly might become very thin, due to flow of the braze into interfaces
containing titanium.
Preferred amounts of silver, copper and titanium per percentage weight for
each braze 20 to 26 are shown below in Table 1. Preferred thickness in
thousandths of an inch for each braze 20 to 26 before brazing is also
shown in Table 1 (the braze thickness does not include the thickness of a
titanium coating). Ranges of percentage weights and thickness are shown in
parentheses.
TABLE 1
______________________________________
Braze
Composition in weight %
Thickness
Braze
Joined Ag Cu Ti in mils
______________________________________
20 metal member 10
69 27 4.0 3
& 1.sup.st Ni member
(67.7 to
(26.3 to
(1.5 to 6)
(3 to 6)
16 71) 27.5)
22 1.sup.st Ni member 16
71 27.5 1.5 8
& moly member
(68.4 to
(26.6 to
(1.5 to 5)
(5 to 10)
14 71) 27.5)
24 Moly member 14
71 27.5 1.5 8
& 2.sup.nd Ni member
(68.4 to
(26.6 to
(1.5 to 5)
(5 to 10)
18 71) 27.5)
26 2.sup.nd Ni member 18
69 27 4.0 6
& ceramic member
(67.7 to
(26.3 to
(3 to 6)
(3 to 8)
12 69.8) 27.2)
______________________________________
The first and fourth brazes 20 and 26 may each include a single
silver-copper braze foil (e.g., BVAg 8, AWS 5.8) that is coated with
titanium. The second and third braze foils 22 and 24 may each include the
following two braze foils to obtain the desired thickness and lower
titanium content: a silver-copper braze foil (e.g., BVAg 8, AWS 5.8) and a
silver-copper braze foil (e.g., BVAg 8, AWS 5.8) coated with titanium.
The metal member 10, the ceramic member 12, the molybdenum member 14, the
nickel members 16 and 18 and the brazes 20 to 26 may be assembled using
standard brazing cements and fixtures (block 104). This assembly may be
placed in axial compression at a low level (e.g., 2.5 psi).
The assembly is placed in a controlled atmosphere furnace, where brazing is
then carried out in a single step (block 106). For example, the brazing
step may be carried out under vacuum at a brazing temperature between
840.degree. C. and 950.degree. C. for between five and thirty minutes. The
brazing causes the brazes 20 to 26 to melt. The assembly is removed from
the furnace and allowed to cool to room temperature (block 108).
The presence of titanium in each braze 20 to 24 limits the migration of
braze during the brazing operation. Small amounts of braze might migrate
and wet the sides of the molybdenum or nickel interlayer members 14, 16
and 18. Consequently, the braze joint might exhibit a range of thickness
after brazing.
Reference is now made to FIG. 3, which illustrates the metal member 10, the
ceramic member 12 and the interlayer after brazing. FIG. 3 is merely an
illustration; the layers are not shown to scale. For the brazes 20 to 26
and members 14 to 18 described in Table 1 and joined as described above,
the first braze 20 will have a thickness of between about 0.003 and 0.006
inches after brazing, the second and third brazes 22 and 24 will each have
a thickness of between about 0.005 and 0.008 inches after brazing, and the
fourth braze 26 will have a thickness of about 0.006 to 0.010 inches after
brazing.
The thicker braze layers at the molybdenum-nickel and ceramic-nickel
interfaces allow for additional stress relief during cooling from the
brazing temperature. Ceramic-metal interfaces and molybdenum-metal
interfaces experience larger stresses during cooling after braze
solidification due to the larger difference in thermal expansion
coefficient and limited ductility of these materials.
Thus disclosed is a method of creating a braze joint that is strong under
bending and torsion. The molybdenum member 14 provides a high elastic
modulus, low thermal expansion buffer between the metal member 10 and the
ceramic member 12. The crystal axis of the molybdenum member 14 is
oriented such that the boundaries are relatively stress-free during
torsion and bend loading of the joined metal and ceramic members 10 and
12. The molybdenum member 14 may be plated with nickel to enhance wetting
and flow of braze and to provide a clean surface for brazing.
The braze joint may be formed in a single brazing step instead of two
steps. The use of a single step reduces cost and complexity of joining the
metal to the ceramic.
The method of joining metal to ceramic may be applied to different types of
turbomachines having rotating ceramic and metal components. For example,
FIG. 4 shows a turbomachine 200 having a ceramic wheel 202 that is joined
to a metal shaft 204 by a braze joint 206 created according to the method
described above. The wheel 202 may be made of a ceramic such as silicon
nitride, and the shaft 204 may be made of a high temperature alloy such as
Monel K500 or a hardened steel such as 4340 steel.
In a turbomachine such as an automotive turbocharger, the braze joint can
transmit high torque loads without the use of a metal sleeve. This
eliminates problems such as procuring special, hard-to-find metal alloys
used for the sleeve. Eliminating the sleeve reduces the need for precision
machining of ceramic and sleeve mating surfaces. Eliminating the sleeve
also reduces the cost of sleeve material, and it makes braze joint
inspection easier to perform.
The present invention is not limited to the specific embodiments disclosed
above. The composition of the metal member 10 steel is not limited to
Monel K500 or 4340 steel, and the composition of the ceramic member 12 is
not limited to silicon nitride
The molybdenum member 14 could be plated with a material other than nickel.
For example, the molybdenum member 14 could be plated with nickel-copper
or another plating material that is compatible with molybdenum.
The brazes 20 to 26 are not limited to copper-silver-titanium materials.
For example, copper-silver-indium-titanium brazes may be used instead.
The nickel members 16 and 18 can be joined to the molybdenum member 14 in
ways other than brazing.
Moreover, the ductile members 16 and 18 are not limited to a nickel
composition. Rather, the ductile members 16 and 18 may be made of any
other metal that can be joined to the molybdenum member 14, that can be
joined to the metal and ceramic members 10 and 12, and that can provide
suitable stress relief during large thermal expansion differences between
the metal, ceramic and molybdenum members 10, 12 and 14.
Thus, the present invention is not limited to the specific embodiments
disclosed above. Instead, the present invention is construed according to
the claims that follow.
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